Added challenge #5

Challenges/#5 - ufn_DaysSinceLastInvoice.sql

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+-- Requirement: ufn_DaysSinceLastInvoice
+
+-- Our sales team often wants to understand the purchasing patterns of our customers. Specifically, they're interested in knowing, for a given invoice, 
+-- how many days had passed since the last invoice for that same customer. This would help us identify customers with consistent purchasing frequencies 
+-- as well as those whose purchasing has deviated from their usual patterns.
+
+-- Function Objective:
+-- Create a tabular function that, when provided an invoice ID, returns the number of days since the last invoice for that same customer.
+
+-- Function Specifications:
+
+-- Function Name: ufn_DaysSinceLastInvoice
+
+-- Input Parameter: InvoiceID (integer)
+
+-- Output Columns:
+-- CurrentInvoiceID: The provided InvoiceID.
+-- CustomerID: The ID of the customer associated with the provided InvoiceID.
+-- PreviousInvoiceDate: The date of the last invoice for that customer before the provided InvoiceID.
+-- CurrentInvoiceDate: The date of the provided InvoiceID.
+-- DaysSinceLastInvoice: The difference in days between CurrentInvoiceDate and PreviousInvoiceDate.
+-- Logic:
+
+-- For a given InvoiceID, retrieve the customer associated with it.
+-- Find the date of the last invoice for that customer that occurred before the provided InvoiceID.
+-- Calculate the difference in days between the provided InvoiceID's date and the last invoice's date.
+-- The function should handle scenarios where there is no previous invoice for the customer (i.e., it's their first purchase). In such cases, the PreviousInvoiceDate 
+-- should be NULL, and DaysSinceLastInvoice should also be NULL.
+
+-- Solution: WideWorldImporters\Challenges\Functions\ufn_DaysSinceLastInvoice.sql
+
+SELECT 
+    i.InvoiceID
+    , i.CustomerID
+    , l.PreviousInvoiceDate
+    , l.CurrentInvoiceDate
+    , l.DaysSinceLastInvoice
+FROM 
+    Sales.Invoices AS i
+CROSS APPLY 
+    dbo.ufn_DaysSinceLastInvoice(i.InvoiceID) AS l
+WHERE 
+    i.CustomerID = 847;

WideWorldImporters/Challenges/Functions/ufn_DaysSinceLastInvoice.sql

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+CREATE FUNCTION dbo.ufn_DaysSinceLastInvoice (@InvoiceID INT)
+RETURNS TABLE
+AS
+RETURN 
+(
+    SELECT
+        CurrentInvoice.InvoiceID AS CurrentInvoiceID
+        , CurrentInvoice.CustomerID
+        , PreviousInvoice.InvoiceDate AS PreviousInvoiceDate
+        , CurrentInvoice.InvoiceDate AS CurrentInvoiceDate
+        , DATEDIFF(DAY, PreviousInvoice.InvoiceDate, CurrentInvoice.InvoiceDate) AS DaysSinceLastInvoice
+    FROM
+        Sales.Invoices AS CurrentInvoice
+    LEFT JOIN
+        Sales.Invoices
+            AS PreviousInvoice
+        ON CurrentInvoice.CustomerID = PreviousInvoice.CustomerID
+            AND CurrentInvoice.InvoiceDate > PreviousInvoice.InvoiceDate
+    WHERE
+        CurrentInvoice.InvoiceID = @InvoiceID
+        AND (
+            PreviousInvoice.InvoiceDate IS NULL
+            OR PreviousInvoice.InvoiceDate
+            = (
+                SELECT MAX(InvoiceDate)
+                FROM Sales.Invoices
+                WHERE CustomerID = CurrentInvoice.CustomerID
+                    AND InvoiceDate < CurrentInvoice.InvoiceDate
+            )
+        )
+);